Papillary thyroid cancer (PTC) and follicular thyroid cancer (FTC) are the most common thyroid malignancies and a subgroup of them can become aggressive and incurable, creating significant prognostic and therapeutic challenges. Our group has made an award-winning landmark discovery of TERT promoter mutations in thyroid cancer (Liu X et al. Endocr Relat Cancer 2013;20:603-610) and has recently published two more articles partially documenting the pathologic role and clinical relevance of these mutations in thyroid cancer (Liu et al. J Clin Endocrinol Metab 2014; Xing et al. J Clin Oncol 2014). In this early phase of this exciting new research area, however, much is unknown about the TERT promoter mutations; their pathologic roles and clinical significance in thyroid cancer remain to be fully defined and the molecular mechanisms remain to be elucidated. Based on our strong preliminary data, we hypothesize that TERT promoter mutations play an important role in the tumorigenesis and aggressiveness of thyroid cancer, particularly when in cooperation with classical driver genetic alterations activating the MAP kinase pathway in PTC and the PI3K pathway in FTC, which is further modified by a common polymorphism rs2853669 T>C in the TERT promoter; this has two important clinical implications, 1) co-existence of TERT promoter mutations with classical genetic alterations, depending on the rs2853669 status, may be associated with particularly poor outcomes of PTC and FTC and therefore has a unique prognostic value; and 2) therapeutically targeting both TERT and the signaling pathway harboring the co-existing genetic alterations in such thyroid cancer may be particularly effective. We propose to pursue the following three Specific Aims to test this hypothesis: 1) To extensively explore TERT promoter mutations 1,295,228 C>T and 1,295,250 C>T and SNP rs2853669 in PTC, their relationship with classical genetic alterations in the MAPK pathway and clinicopathological outcomes, and their prognostic value for PTC; 2) To similarly explore these TERT promoter variants in FTC, their relationship with genetic alterations in the PI3K pathway and clinico-pathological outcomes, and their prognostic value for FTC; and 3) To use in vitro and in vivo approaches to functionally test the role of TERT promoter variants in cooperation with classical oncogenes as well as the mechanistic role of the EST system in thyroid tumorigenesis and the therapeutic potential of targeting them. These studies are among our strongest research areas and will likely have a major impact on the field of thyroid cancer by unveiling new genetic patterns and mechanisms, based on which novel prognostic and therapeutic strategies may be developed for thyroid cancer.